Point-to-point data communication over coaxial or fiber optic media can be implemented using a general purpose interface such as the AMD AM7968 taxi chip transmitter and AM7969 taxi chip receiver. These devices, whose characteristics are described in AMD Publication No. 07370, issued May 1987 and thus their components and functions will not be explained in detail in the specification, provide for very high speed point-to-point communications by loading data into the input side of the transmitter chip, and outputting it on the output side of the receiver chip, the chips being separated by a serial cable of indeterminate length. A complete link suitable for full-duplex communication would consist of two such serial data links, one for each direction. In a typical data communication scheme, data is transmitted over the serial cable using square wave pulses having different phases, each transition of the transmitted chips output signal corresponding to a one, and the lack of a transition corresponding to a zero. Thus, for example, referring to FIG. 1 at line a, if the objective is to transmit a bit stream comprising 101100111, then as shown at line b, each 1 is represented as a transition. The direction of the transition in this scheme is not relevant.
It has previously been realized that transmission of a long string of zeros would present a problem for the receiver, as the receiver must periodically see a transition in order to adjust the rate at which it samples the incoming data. An absence of transitions for too long a period of time will allow the receiver to drift because of an absence of inputs to the phased lock loop incorporated in the receiver channel, causing a loss of synchronization between the receiver and the transmitter. Therefore, the transmitting chip incorporates means for implementing a run length limited (RLL) coding technique that allows transmission of 100 megabit per second data on a 125 megabit per second channel. Each 8-bit byte to be transmitted is divided into two 4-bit nibbles. Each nibble is then transmitted as a 5-bit symbol, the 5-bit symbol being defined according to a coding scheme which will prevent the transmission of more than three successive zeros at any time in the transmitted data stream.
While there are 16 combinations of each 4-bit nibble that is to be encoded, there are 32 possible combinations of resultant 5-bit code. Therefore, in addition to minimizing the number of consecutive zeros in the output code, the output codes also have the characteristic that a minimum number of zeros appear at either end of the 5-bit output.
The maximum time between transitions is 3-bit times, and the minimum time between transitions being 1-bit time (a bit time being defined as 8 nanoseconds). Considering the actual frequencies of the signals on the wire, since the transmission rate is 125 megabits per second, the frequency is half of that or 62.5 megahertz at its maximum rate, the minimum rate being one fourth of that or about 13.1 megahertz.
In known data transmission schemes, two types of links may be constructed. One uses fiber optics which allows signal transmission over long distance with minimal attenuation, but at high cost. A second type also uses the above-described data transmission protocol, but using inexpensive coaxial cable as the transmission medium.
The problem to be addressed by the present invention is the potential loss of accuracy when a signal encoded according to the method described above is transmitted on coaxial cable. The result of such transition is attenuation of the transmitted signal. It is known that the signal will be distorted since the cable behaves as a low pass filter. Low frequency components (strings of zeros, without transitions) reach higher (positive or negative) amplitudes than high frequency components (strings of ones, having transitions at an 8 nanosecond rate). For a sufficiently long cable having greater attenuation, a pattern consisting of . . . 00011000 . . . cannot be reliably detected by simply comparing the voltage on the cable with a fixed slicing level (typically zero). This is because the initial zeros cause the signal to reach an amplitude that is greater than the amplitude of the opposite going pulse caused by two adjacent ones, the succession of two transitions at an 8 nanosecond rate preventing the pulse from reaching an amplitude which can be easily and reliably detected by a nominally established slicing level or threshold comparison.
One approach to this problem of overcoming the effects of passage of the information through a long cable would be preferential amplification of the high frequencies, using schemes similar to the precompensation utilized in recording techniques.
However, such a scheme would only work for one given length of cable, because the attenuation of the high frequency signal components of the transmitted data code varies with the length of the cable through which the transmission passes.
An alternative approach would be to use a technique wherein instead of looking at the magnitudes of the signals, the detection system looks at the transitions of the signals as is done in magnetic disc recording. The difficulty with implementing this approach in a coaxial cable transmission scheme is that the frequency of transmission under consideration here is orders of magnitude faster than the rate at which data is being read in a magnetic recording scheme.